"It may seem strange to have two opposing signals in the same cell," says the study's senior author, King-Wai Yau, Ph.D, a professor in the Solomon H. Snyder Department of Neuroscience at Hopkins, "but the unique mechanism renders these parietal photoreceptors most active at dawn and dusk."
"So incorporating two different pigments and two separate signaling molecules in one cell may have been an economical way, in a primitive eye with relatively few cell types, to tell the transitions of the day based on changes in the spectrum of sunlight," says Chih-Ying Su, Ph.D., the first author of the study and a former neuroscience graduate student at Hopkins.
"It's just like in a small company," says Yau. "You have to delegate each person to do more things."
By sharing features found in human photoreceptors as well as those found in simpler organisms like the scallop, the researchers propose that the lizard's parietal eye photoreceptor cells represent a "missing link" between the light-sensing apparatus in lower animals and ours.
It turns out that some frogs and fish also have a spot on their foreheads that might play the role of a light-sensing third eye. Yau hopes to pursue these structures to obtain more clues about how our photoreceptor cells, the rods and cones, came about. As he says, he's most curious about how the same function can be achieved in different ways in different animals.